1. Field of the invention:
The present invention relates to a developing unit used in an image forming apparatus such as an electrophotographic copying machine or the like, and more particularly, to a developing unit which utilizes a two-component developer consisting of toners and carriers.
2. Description of the prior art:
In an electrophotographic copying machine, an electrostatic latent image formed on a photoconductor is developed with toners into a toner image by means of a developing unit. FIG. 3 shows one example of a developing unit which utilizes a two-component developer consisting of toners and carriers. Usually, in such a developing unit, the developer is carried on the outer circumferential surface of a developing sleeve 52 disposed facing a photoconductor drum 10, and the toners contained in the developer being carried are made to adhere to an electrostatic latent image on the photoconductor drum 10 as the developing sleeve 52 revolves, thus developing the latent image into a toner image. Since the toners in the developer in the developing unit are gradually consumed as they are used for developing electrostatic latent images, new toners are supplied as needed to the developing unit from a toner hopper 30.
In the developing unit, an agitation roller 54 is disposed beneath the toner hopper 30 in order to adequately agitate and mix the supplied toners into the developer accommodated in the developing unit. Disposed between the agitation roller 54 and the developing sleeve 52 is a mixing roller 56, above which a partition plate 57 is disposed. There is also disposed, above the developing sleeve 52, a doctor blade 58 which is used to regulate the amount of developer carried on the rotating surface of the developing sleeve 52. The developer stopped by the doctor blade 58 is transported along the upper surface of the partition plate 57 toward the upper portion of the agitation roller 54. The toners supplied from the toner hopper 30 are agitated and mixed with the developer by means of the agitation roller 54 and the mixing roller 56. The rotating direction of the agitation roller 54 disposed beneath the toner hopper 30 is usually set in such a way that the developer flowing down along the upper surface of the partition plate 57 runs counter to the flow of the developer being carried on the rotating surface of the agitation roller 54. The rotating direction of the agitation roller 54 is also set in the same manner mentioned above, even if the mixing roller 56 is not provided in the developing unit and only the agitation roller 54 is disposed beneath the toner hopper 30.
In the developing unit having the above-mentioned construction, since the developer consisting of toners and carriers accommodated in the developing unit has a greater specific gravity than the toners supplied to the developing unit, the supplied toners cannot be easily mixed into the developer being carried on the rotating surface of the agitation roller 54, and may therefore remain afloat over the developer existing in the upper portion of the agitation roller 54, especially when the flowing speed of the developer on the rotating surface of the agitation roller 54 is not fast enough, or the developer taken up by the agitation roller 54 exhibits a flowing movement such as shown by an arrow F, or when the developer above the rotating surface of the agitation roller 54 is pushed by the developer flowing down from the partition plate 57 and forced in the opposite direction to that of the rotation of the agitation roller 54. Thus, when the toners keep afloat over the developer, the toner density with respect to the developer in the developing unit may not increase substantially, but the final image density may decrease. Also, since the toners are supplied by rotating a toner supply roller 31 disposed in the toner hopper 30 when the toner density in the developing unit decreases, if the supplied toners remain afloat over the developer being carried on the agitation roller 54, it is possible that the toner density may abruptly increase after the completion of the toner supply, thus resulting in the final image being fogged. Furthermore, if the developing unit is operated after it is once stopped with the supplied toners remaining afloat over the developer on the surface of the agitation roller 54, the operation of the developing unit will cause the toners floating over the developer on the surface of the agitation roller 54 to be mixed into the developer, resulting in a sudden increase in the toner density. This causes such problems as splashing of toners around the developing unit and changes in the final image density during a continuous copy operation.
If the agitation roller 54 is made to rotate in the opposite direction, the supplied toners can be prevented from remaining afloat over the developer carried on the rotating surface of the agitation roller 54, making it easier for the toners to be agitated and mixed into the developer, but this in turn causes another problem in the reduction in agitation efficiency because the developer flowing down along the upper surface of the partition plate 57 does not run counter to the flow of the developer being carried on the rotating surface of the agitation roller 54.